r75/6 cutting out at 5000 rpm

[gmoreau]

Hello all, I seem to be having an issue with my 1975 R75/6 cutting out at about 5000 RPM,
the carbs are clean with good diaphragms, the points and cond. are new, I thought it might have been the coils so I replaced them with new ones
from EME (endurolast EDL-Coil6Vx2) I have set the points and timing to spec, I do have a bit of differential due to the points follower / cam bent issue, but I have set that to average out the differential. ( one one side the s mark is at the top of the window and on the other the bottom. and I am running the stock BPR6ES plugs. I'm running fresh 93 no ethanol gas. and the valves have been set to spec. Also when I rev it with the cover off I can see the "advance wings" coming out to advance timing. The bike starts beautify, and runs strong UNTILL it hits 5000 RPM and them all most acts like it hits a rev limiter. Any ideas of what to look at next?\\
Thanks Jerry

 

[20774]

Welcome to the forum! Frustrating to try to locate something intermittent, but it sounds like you have a way to reproduce the situation. What does it do specifically? Can you go past 5000 RPM at all, or are you held up at that point? Can you downshift and get to the same RPM or to the same speed as before? Same problem? What about the air filter...could it be holding the carb back? Also, try carefully loosening the gas cap and see if the situation improves. Could be building up a vacuum in the tank.

Also, are you running the stock settings in the carb in terms of jets and needle position?

 

[omega man]

Again, welcome to the forum!
I have seen this if the spark plug wires had “broken down”.
Good luck.
OM

 

[gmoreau]

Welcome to the forum! Frustrating to try to locate something intermittent, but it sounds like you have a way to reproduce the situation. What does it do specifically? Can you go past 5000 RPM at all, or are you held up at that point? Can you downshift and get to the same RPM or to the same speed as before? Same problem? What about the air filter...could it be holding the carb back? Also, try carefully loosening the gas cap and see if the situation improves. Could be building up a vacuum in the tank.

Also, are you running the stock settings in the carb in terms of jets and needle position?

Thank you 20774,
gearing does not matter, sitting on the center stand in the shop , it starts sputtering right at 5k, or on the highway in any gear. gas flows freely, I can remove the bowls and it will flow all day. new petcocks and gas cap, air filter and hoses . carbs are all stock to the best of my knowledge, This one has me stumped

Jerry

 

[20774]

So many things have been changed. Since it all happens at 5K RPM, seems to me that points to the ignition or fuel. At 5K, you're likely running almost totally on the main jet in the carb. Are they stock and/or could they be partially plugged? If the engine cutout is only on one cylinder, then I'd lean more to ignition.

Along the lines of what OM said, I wonder if you could put the bike in the garage or do this at night. With the front cover off (remove the transmission battery ground first), do you see any sparking from the alternator area or maybe the wires when running at 5K completely in the dark?

 

[m_stock10506]

Limited RPM at 4k - 5K is often the diaphragms. Would be worth pulling them out and checking them again. Just because they are new doesn't guarantee one didn't tear. You also said you had, BPR6-ES plugs. Those are the resistor plugs. Did you change the caps to zero resistor caps? Is it possible that too high resistance in the plug wire circuit is limiting RPM? Can you swap out for non-resistor plugs?

 

[PGlaves]

This screams a diaphragm problem. Maybe torn or maybe a vacuum leak in the diaphragm chamber limiting the carb slide travel.

 

[gmoreau]

I think I might have found the issue, following omega man advice , I investigated the plug wires, the connection between the wire and the plug boot did not look great I cut about 3/8 in off the end of the wire and re-seated the wire, and seems to have fixed it, it was late and did not do a road test yet, but reving the bike on the stand seems to confirm the repair. I will update once I have done a road test.

Thanks to all who helped.

Jerry

 

[ebeeby]

Did it pass the road test?